1. Field of the Invention
The present invention relates to synthesis of ultrahigh-purity (UHP) polycrystalline silicon carbide (SiC) for the use as a vapor source in industrial-scale growth of high quality SiC single crystals by sublimation.
2. Description of Related Art
Hexagonal 4H and 6H polytypes of silicon carbide possess unique combinations of electronic and thermo-physical properties, which enable operation of semiconductor devices at significantly higher power, frequency and temperature than comparable devices made from conventional silicon. Semi-insulating (SI) 4H SiC and 6H SiC wafers serve as lattice-matched substrates in GaN-based high-electron-mobility transistors (HEMT) operational at microwave frequencies and high power levels. To provide for optimum device performance, the SiC substrate must have the correct resistivity. For microwave devices, the SiC substrate must be semi-insulating with the resistivity on the order of 1010-1011 Ohm-cm. In order to achieve this resistivity, the presence of unwanted residual impurities in the crystal must be minimized.
Commercial-size SiC single crystals are grown by the sublimation technique called Physical Vapor Transport (PVT). In PVT growth, a graphite crucible, typically a cylindrical graphite crucible, is loaded with polycrystalline SiC source material (typically SiC grain) at the bottom, while a SiC single crystal seed wafer (or segment thereof) is disposed at the crucible top, for instance, attached to the crucible lid. The loaded crucible is placed in a gas-tight furnace chamber and, in the presence of an inert atmosphere, is heated to the temperature of SiC sublimation growth, typically, between 2000° C. and 2400° C., with the temperature of the polycrystalline SiC source material being higher by 10-100° C. than the temperature of the SiC single crystal seed. Under these conditions, the SiC source material sublimes and the sublimation vapors migrate, under the influence of temperature difference between the SiC source grain and the SiC single crystal seed, to the SiC single crystal seed where the vapors condense on the SiC single crystal seed causing growth of a SiC single crystal on said SiC single crystal seed. In order to control the growth rate and ensure high crystal quality, PVT growth is carried out under a small pressure of inert gas, generally, between 1 Torr and 100 Torr.
Availability of high-purity SiC source material is important for the growth of SiC single crystals in general, and it is crucial for semi-insulating SiC crystals. In addition to high purity, the SiC source material must be of proper polytype and grain size. For the growth of 6H and 4H SiC single crystals, the desired SiC source material is of “alpha” polytype, that is with crystallites belonging to SiC hexagonal polytypes, such as 4H and/or 6H.
Prior art SiC synthesis includes four basic methods. The methods are:
The Acheson Process
The most widely used large-scale commercial process for production of technical grade SiC is disclosed in U.S. Pat. Nos. 492,767 and 615,648. In this process, a mixture of quartz sand (SiO2) and coke (C) containing various additives is heated up to 3000° C. to form a mass of SiC crystals according to the reaction: SiO2+3CSiC+2CO. While numerous modifications of the Acheson process have been developed over the years, the produced SiC material always contains high concentrations of boron, nitrogen, aluminum and other metals, and is unsuitable as a source material for the growth of semiconductor-quality SiC crystals.
Chemical Vapor Deposition (CVD)
Bulk SiC shapes with a density close to a theoretical SiC density (3.2 g/cm3) are produced commercially by CVD (see for instance U.S. Pat. No. 5,704,985). In this process, silicon and carbon-containing gaseous precursors react at elevated temperatures, typically, 1200° C. to 1400° C., to form solid SiC. Commonly, SiC is deposited on a suitable substrate such as graphite. A single precursor containing both Si and C atoms, such as Trimethylsilane, can be used as well. Although high-purity precursors are available, commercial-grade bulk SiC produced by CVD is not pure enough for the use as a source in SiC crystal growth, especially for semi-insulating SiC crystals, as such commercial-grade bulk SiC usually contains boron (0.7-2 ppm), metal impurities and nitrogen (up to 100 ppm). In addition, the CVD process yields cubic “beta” polytype of SiC, which is undesirable for crystal growth of 4H and 6H SiC polytypes.
Reactions Between Liquid or Solid Silicon and Carbon Compounds
U.S. Pat. No. 5,863,325 is an example of this approach to SiC synthesis, wherein organic alkoxysilanes and inorganic SiO2 were used as a Si source, while phenolic resin was used as a C source. This type of reaction requires catalysts and other additives, which are undesirable from the standpoint of purity. The produced SiC material contains large concentrations of contaminants and is unsuitable for the growth of semiconductor-quality SiC crystals.